The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
An electromagnetic beam with subwavelength beamwidth and low sidelobes is important to the operation of a wide array of electromagnetic devices. High resolution probes, sensors and imaging systems, high density data storage devices, lithography systems, biomedical targeting devices, and wireless power transfer systems are a few examples of such devices.
Although subwavelength electromagnetic confinement is necessary for these devices, electromagnetic confinement can only be obtained at extremely close distances, due to the diffraction limit. Stated differently, such confinement is only possible at extremely close distances where evanescent components exist. Such confinement requirement is difficult to meet in many applications and limiting for many devices. For example, a subwavelength probe placed in close proximity of a biomedical sample can distort both the sample and the measurement results. As a result, there is growing interest in a method that can overcome the diffraction limit at extended operating distances.
Over a decade ago, metamaterial superlenses were proposed as a possible solution by J. B. Pendry, Phys. Rev. Lett., 85, 3966, 2000. Metamaterial superlenses can enhance and recover the evanescent spectrum to overcome the diffraction limit over an extended range. Accordingly, they held great promise for improving the performance of near-field devices and their proposal was followed by numerous publications. Several metamaterial superlenses were experimentally demonstrated and their ability to overcome the diffraction limit and obtain super-resolution was verified. However, the proposed metamaterial superlenses were limited by loss, narrow frequency bands of operation, polarization dependence, and fabrication challenges.
More recently, an alternative method to overcome the diffraction limit has been proposed, which relies on the interference of highly oscillatory electromagnetic fields to form subwavelength patterns as discussed by R. Merlin, Science, 317, 927, 2007 and U.S. Pat. No. 8,003,965. Such highly oscillatory evanescent fields are realized using near-field plates (NFPs). NFPs are non-periodically patterned surfaces, which are designed to form a prescribed subwavelength focal pattern at a specified focal plane. NFPs have demonstrated several advantages over metamaterial superlenses. Firstly, NFPs are patterned surfaces or arrays which are much simpler to fabricate compared to volumetric metamaterial superlenses. Secondly, NFPs have been shown to be robust to practical losses. Thirdly, NFPs allow one to stipulate the subwavelength near-field focal pattern, a unique feature not offered by metamaterial superlenses. Fourthly, the design of NFPs is scalable with frequency. For example, NFPs have been pursued from kilohertz to optical frequencies.
While previous NFPs have demonstrated extreme field tailoring capability to form subwavelength focal patterns, their performance was limited by undesired fields in directions other than the direction of the subwavelength focus or field maximum. This issue can limit the utility of NFPs in practical applications. For example, an NFP which is used as a probe to detect objects in its focal plane may couple to objects in locations other than the focal plane. Thus, it is desirable to have an NFP that concentrates radiation from a source into subwavelength dimensions without reflection. In other words, the NFP should form a “unidirectional” subwavelength near-field pattern.